A sump is a reservoir in which water is stored or collected. Therefore, sumps could be in the form of water tanks, ponds, lakes or any other natural or artificial water body. Water is removed from the sump with the help of a pump, typically a sump pump, which can be a centrifugal pump, a reciprocating pump, a pedestal pump, a submersible pump, an ejector pump and the like. The pump is usually installed in a pit which is dug to a level that is close to bed of the sump. Generally a draft tube is provided at the inlet of the pump for directing water from the sump. After the pump is started, due to a pressure differential at both ends of the draft tube, water is drawn from the sump into the draft tube towards the inlet of the pump. As the water is pumped, the water level in the sump starts decreasing. The suction force creates vortices in the water mass in the sump. When the water reaches a certain submergence level, the critical submergence, due to the suction force exerted by the pump the surface of the water is broken, allowing atmospheric air to be sucked into the mass of the water and particularly the vortices. This air enters the draft tube in the form of voids and bubbles. This phenomenon is called air entrainment. The air entrainment imparts uneven loading on the impeller eye of the pump, effecting vibrations in the pump, thereby causing the pump to de-prime.
In order to avoid air entrainment damaging the different parts of the pump, a close monitoring over the submergence level is inevitable. Therefore, when the submergence level comes down to a point where air entrainment begins, the pump is required to be shut down. This monitoring can be achieved by physically measuring the critical submergence level and then shutting the pump when critical submergence level is reached or by employing sensors or floating switches that automatically shut down the pump at the critical submergence level. The water below the critical submergence level cannot be pumped which leads to wastage of that water due to non-usability.